In recent years, the importance of radio based telephony has increased rapidly. Among the many reasons for this is the added flexibility for the user, and relatively high costs of installing the cables needed in the fixed telephone network. A radio based telephone system consists of so called cells, each of which receives radio coverage from a certain radio base station. The base stations are in turn connected to the fixed telephone network through a special switching node. A typical cellular communication system is illustrated in FIG. 1.
FIG. 1 illustrates ten cells, the C1-C10 in a typical cellular mobile radio communication system. Normally, a cellular mobile radio system would be implemented with more than ten cells. However, for the purposes of simplicity, the present invention can be explained using the simplified representation illustrated in FIG. 1. For each cell, C1-C10, there is a base station, B1-B10, with the same reference number as a corresponding cell. FIG. 1 illustrates the base stations as situated in the vicinity of the cell center and having omnidirectional antennas. The base station could also be situated at a cell border and use directional antennas.
FIG. 1 also illustrates nine mobile stations, M1-M9, which are movable within a cell and from one cell to another. In a typical cellular radio system, there would normally be more than nine cellular mobile stations. In fact, there are typically many times the number of mobile stations as there are base stations. However, for the purposes of explaining the present invention, the reduced number of mobile stations is sufficient.
Also illustrated in FIG. 1 is a mobile switching center MSC. The mobile switching center MSC is connected to all ten base stations B1-B10 by cables. The mobile switching center MSC is also connected by cables to a fixed switched telephone network or similar fixed network. All cables from the mobile switching center MSC to the base station B1-B10 and cables to the fixed network are not illustrated.
In addition to the mobile switching center MSC illustrated, there may be additional mobile switching centers connected by cables to base stations other than those illustrated in FIG. 1. Instead of cables, other means, for example, fixed radio links, may also be used to connect base stations to mobile switching centers. The mobile switching center MSC, the base stations, and the mobile stations are all computer controlled.
As the popularity of cellular communications systems increases, the existing cellular systems become more and more crowded. As a result, it is desirable to increase the range and/or capacity of the cellular system. Furthermore, it is desirable to reduce the cost of new cellular communication systems. One way to lower costs is to use fewer base stations to cover a certain area. However, as the range of each base station is expanded, the sensitivity of each base stations' receiver must be increased if the mobile station output power is unchanged.
The current digital cellular systems employ base stations which separate mobile signals using time and frequency orthogonality. Signals from a mobile station propagate to a base station and the signals are received at a single or sometime double antenna which are closely spaced, e.g., approximately 20 wavelengths. The receiver processes the signal using time and frequency orthogonality to separate signals from different users. While techniques such as frequency hopping and advance coding techniques provide ways for lowering co-channel interference, these techniques are inherently limited by the available frequency spectrum. However, the use of directional sensitivity of adaptive antennas offers a new way of reducing co-channel interference. An adaptive antenna consists of an array of spatially distributed antennas. Impinging on the array are signals from a plurality of transmitters. By properly combining the antenna outputs, it is possible to extract individual signals from the received superposition, even if they occupy the same frequency band. Furthermore, a beamforming matrix can be used to shape the reception patterns for the antenna array. As a result, the beamforming matrix has a plurality of outputs each corresponding to a section of the cell. The best combination of outputs is then used when analyzing the detected signals.